In the foundry art, molds and cores used in making metal castings are prepared from an aggregate such as sand and a curable binder. In recent years, the curable binders of the no-bake type, i.e., those requiring little or no additional heat other than that available at ambient temperatures, have gained wide acceptance in the foundry art. Typically, to be useful as a no-bake binder, the system must have a sufficient bench or work life to permit shaping of the foundry mix, must rapidly develop good tensile strength when cured at room temperature and must provide rapid stripping of the cores. In addition, the binder system should work equally well on all foundry aggregates, be relatively insensitive to moisture and give castings without surface defects with all metal types.
Phenol-formaldehyde resins have been crosslinked with diisocyanates for use in the coatings and adhesives industries, and phenol-formaldehyde resins crosslinked with isocyanates have been used in the foundry process for mold and core making.
U.S. Pat. Nos. 3,409,579; 3,432,457; 3,485,797; 3,676,392; 3,702,316; and 3,726,867 teach the use of special phenol-formaldehyde resins which contain benzylic ether linkages (FIG. I, portion m) which are prepared at temperatures in excess of 100.degree. C. with special divalent metal ion catalysts. ##STR1##
These special resins are described as high molecular weight phenolic resins containing more benzylic ether linkages than methylene linkages between the phenol rings, i.e., m is greater than n in FIG. I, and the number of repeating aromatic rings is at least 3 and preferably 4 to 10. In addition these resins are described as being essentially anhydrous in that they contain less than 5% and preferably less than 1% water.
Phenolic resoles are normally prepared using an alkaline catalyst such as sodium hydroxide with a molar ratio of formaldehyde to phenol greater than one. This reaction can be divided into two temperature ranges; (1) reaction temperatures in excess of 90.degree. C., which yields conventional resole resins containing methylene bridges (FIG. II), ##STR2## wherein the average n is typically 3 or more, and (2) reaction temperatures less than 90.degree. C. The latter temperature condition will yield low molecular weight materials where the degree of advancement will depend on the actual temperature used and the length of the reaction. The molecular weight of these low temperature alkaline-catalyzed one-step phenolic mixtures is a function of the degree of reaction. These materials may range from mixtures of mononuclear methylolated phenols of the type shown in FIG. III with molecular weights of 125-150, and up to resins with molecular weights of 1,000 or greater, depending upon time of reaction. ##STR3##
Binder systems based on such low molecular weight methylolated phenols are described in U.S. Pat. No. 4,148,777.
Binder systems based on resole phenolic resins containing at least 5% water, preferably combined with polyether polyols, crosslinked with polyisocyanates are described in U.S. Pat. No. 4,079,031.
The resole resins and the benzylic ether resins of the prior art phenolic urethane foundry binder systems required the use of hydrocarbon solvents and generally required both a polar solvent and a non-polar aromatic hydrocarbon solvent to provide a mutually compatable solvent system for uniform dispersability of the binder system over the aggregate, see for example, U.S. Pat. No. 3,726,867. Protic or aprotic solvents are suitable for phenolic resins, but are unsuitable with the polyisocyanates, which require inert aromatic hydrocarbon solvents. Consequently, in the prior art systems, hydrocarbon solvents were combined with moderately polar protic or aprotic solvents to provide a compatable solvent system to insure adequate miscibility between the polyisocyanate and phenolic binder during the phenolic isocyanate reaction.
The solvent systems employed by the prior art foundry binder systems provide binders which may be readily mixed with the sand aggregates and cured to give molds and cores of reasonably high tensile strengths, but during the foundry process (including the mixing of the binder with the sand, the shaping, the curing, the stripping, the aging, and the metal pouring), the hydrocarbon solvents volatilize to a significant extent. The volatilization of the hydrocarbon solvents during the foundry process creates unpleasant and uncomfortable conditions for the foundry workers in certain areas. While such undesirable working conditions may, at times, be corrected by ventilation systems, the discharge of such hydrocarbon vapors into the atmosphere is also objectionable from an environmental point of view.
The present invention overcomes the problems of the prior art by providing a foundry binder system which is free of pure hydrocarbon solvents and uses in lieu thereof reactive diluents which are reactive with the isocyanate component of the foundry binder system. Moreover, the preferred diluents of the present invention have a relatively low vapor pressure, i.e., lower than the comparable hydrocarbon solvents. As a result of using the selectively reactive diluents, the present invention provides a foundry binder system which may be substituted for existing foundry binder systems with the advantage of substantially lowering the levels of emissions during the foundry process.